1. Field of the Invention
The present invention relates to a process for producing fine oil droplets in capsules having strong protective shells and more particularly it relates to a process for producing fine oil droplets or oil-containing microcapsules having strong protective outer shells by dispersing or emulsifying in a polar solvent a polyisocyanate adduct having a free isocyanate group and adding to the dispersion or emulsion a polyamine or a polyamine adduct having a free amino group as a polymerization promotor, whereby the polymerization of the polyisocyanate adduct is caused from the outer side of each oil droplet to insolubilize the polyisocyanate adduct.
According to the process of this invention, the size of the capsules can be controlled very easily and further the oil-containing capsules or microcapsules prepared by the process of this invention have strong outer shells showing quite a low permeability.
The oil-containing fine capsules or microcapsules produced by this invention may be used for the preparation of pressure-sensitive copying sheets as well as may be utilized for the purpose of generally protecting dyes, inks, perfumes, adhesives, medicaments, etc., in the oil-containing microcapsules and also for preserving therein foods which readily deteriorate.
2. Description of the Prior Art
For encapsulating an oily liquid by dispersing or emulsifying in a polar liquid the oily liquid having incorporated therein a wall- or shell-forming material and insolubilizing the wall-forming material from the outer side of each oil droplet in the dispersion or emulsion by the action of a polymerization promotor, etc., the following processes are known:
(A) A process in which an initial polymerization product for synthetic resin having in the molecule both an oleophilic group and a hydrophilic group so that the molecules to be polymerized accumulate at the water-oil interfaces is dissolved in an oil, the oily liquid thus prepared is dispersed or emulsified in a polar solvent, and then a polymerization promotor is added to the dispersion or emulsion to cause the polymerization of the initial polymerization product from the outer side of each oily droplet in the dispersion or emulsion and to insolubilize there the initial polymerization product (see, Japanese Pat. Publication No. 19,574/'63).
(b) A process in which a hydrophobic liquid containing polyurethane having a free isocyanate group is emulsified as fine droplets in water containing a cyclic diamine, whereby the free isocyanate group containing polyurethane is caused to react with the cyclic diamine at the interfaces between both of the liquids to form an insoluble polymer and encapsulate the oil droplets with the insoluble polymer (see, Japanese Pat. Publication No. 446/'67).
(c) A process in which a hydrophobic liquid containing a polyisocyanate is emulsified in water containing a polyamine to cause the reaction of the polyisocyanate and the polyamine at the interfaces between both of the liquids and encapsulate the droplets of the hydrophobic liquid (see, Japanese Pat. Publication No. 771/'67).
(d) A process in which a hydrophobic liquid containing a prepolymer of a polyisocyanate is dispersed in a hydrophilic liquid containing a polyamine or a polyhydroxy compound, whereby both of the components are reacted at the interfaces between both of the liquids to form a polymer insoluble in both liquids and encapsulate the droplets of the hydrophobic liquid with the insoluble polymer (see, British Pat. Specification No. 1,091,077).
(e) A process in which an oily liquid containing a polyisocyanate monomer is dispersed or emulsified in a polar liquid and then a polyamine or a polyhydroxy compound is added to the dispersion or emulsion, whereby the polyisocyanate monomer is caused to react with the polyamine or the polyhydroxy compound at the interfaces between both of the liquids to form the walls of the capsules (see, British Pat. Specification No. 1,091,141).
Also, a pressure-sensitive copying sheet utilizing the microcapsules disclosed in British Pat. Specification No. 1,091,077 described above is set forth in the specification of Japanese Patent Publication No. 27,257/'69.
In the above-described processes (b), (c), and (d), the oil droplets are formed by adding a polyamine or a polyhydroxy compound to a polar liquid and then adding to the polar liquid an oily liquid containing a polyisocyanate monomer or a polyisocyanate prepolymer. In the process (e), a feature thereof is to use a polyisocyanate monomer.
However, in such known processes it is difficult to form strong and impermeable capsule walls or shells. That is to say, when a polyamine is incorporated in a polar liquid prior to the emulsification as in processes (b), (c), and (d) shown above, an aggregation occurs markedly on emulsification to form polynuclear capsules composed of several or several tens of aggregated oil droplets and in an extreme case the entire liquid is aggregated. Such polynuclear capsules are not very resistant to pressure or friction.
Furthermore, in the afore-described processes a film of a polymer is formed at the surface of the oil droplet during the emulsification, which results in making it difficult to emulsify further and also to control the size of the oil droplets.
In process (e) which employs a polyisocyanate monomer, the capsule walls formed are quite imperfect, the walls are not very resistant to pressure and friction, and the walls show a high permeability, which gives difficulties in the retention of the contents therein.
A first object of the present invention is to provide a process for producing microcapsules in which the size of microcapsules can be controlled very easily.
A second object of this invention is to provide a process for producing microcapsules having quite strong coating or walls.
A third object of this invention is to provide a process for producing microcapsules having coating or walls which are less permeable.
The other objects of the present invention will become apparent from the following descriptions.